The problem is that they've requested me to follow the bad way: it was
required that the data must be stored on the minor, local server
first, and replicated to the central server after. I know it is
multimaster replication and it is dangerous, not kosher and whatnot,
but they insist on using this topology. Also, they don't want to use
some other external application. The problem is that the Internet
links are very unstable here, and expecting a write on the central
server and its posterior replication can stop an office for some days.

I'd setup the central server as a syncrepl consumer to all the branch
offices, with one database context for each. Optionally those can be
glued together with back-ldap. Using the chain overlay on the central
server, writes will be internally redirected to the branch office and
you won't have to deal with referrals manually. Of course, if the branch
is down writes will fail...
With a modern version you should be able to use refreshAndPersist sync
replication with very low delays in replicating the changes back to the
central server.